Process and composition for cleaning and polishing aluminum and its alloys



Jan. 11, 1966 E. w. KENDALL 3,223,816 PROCESS AND COMPOSITION FOR CLEANING AND POLISHING ALUMINUM AND ITS ALLOYS Filed Feb. 21, 1962 2 Sheets-Sheet l FIG. I

as 34 2s 29 3o\ 35 27 l/ /V \\\\\l I/// //X \\\\\\\\\i FIG. 3 FIG. 4

as a? I 60 6I T6 U TREATED W FIG. 7 FIG. 8

INVENTOR.

E. W. KENDALL ATTORNEY Jan. 11, 1966 PROCESS AND COMPOSITION FOR CLEANING AND POLISHING Filed Feb. 21, 1962 606I- T6 TREATED E. w. KENDALL 3,228,816

ALUMINUM AND ITS ALLOYS 2 Sheets-Sheet 2 606I-T6 CONTROL MATERIAL SPECIMEN MINIMUM GAGE MAX/MUM GAGE AVERAGE GAGE DIFFERENCE 202+ CONTROL .03984 .04083 .04026 /-2 TREATED .05974 .04/06 .04026 2-1 CONTROL .04032 .04/50 .04090 7075 T6 2-2 TREATED .04042 .0413/ .0407? 000/3 3-! CONTROL .04074 .0429/ .04/92 3003 0 3-2 TREATED .0413! .04304 04195 00003 4-! CONTROL 04016 .04096 .0406? T6 4-2 TREATED .04038 .04/38 .04083 +000;

5-1 CONTROL .04934 .05040 04909 2024 T6 5-2 TREATED .04976 .05030 .05002 +0005 6-! CONTROL 0/946 .0203? .0200/ 2024 6-2 TREATED .0/952 .0/987 .0197! 00030 7-! CONTROL 040?? .04/63 .04/06 5052 0 7-2 TREATED .04070 .04/44 .04/02 00004 6O (I) 8 4o 6%? I 8 FIG. I2

/ INVENTOR.

E. W. KENDALL I0 M kid/Hon ATTORNEY United States Patent 3,228,816 PRQCESS AND COMPGSITIQN FOR CLEANING AND POLISHING ALUMINUM AND ITS ALLOYS Earl W. Kendall, San Diego, Calif., assignor to Ruhr Corporation, a corporation of California Filed Feb. 21, 1962, Ser. No. 174,838 The portion of the term of the patent subsequent to Oct. 8, 1980, has been disclaimed 16 Claims. (Cl. 156-21) This invention relates generally to the art of treating metals to produce desired surface characteristics such as high polish or gloss, low electrical resistance, and freedom "from metallic oxide films and contaminants. More specifically, the present invention is directed to a process and composition for chemically cleaning and polishing aluminum and the alloys of aluminum by simple immersion in a chemical aqueous solution comprised mainly of minor proportions of nitric, hydrofluoric, and boric acids.

This application is a continuation-in-part of my copending application for Cleaning and Polishing Aluminum and Its Alloys, Serial No. 812,369, filed May 11, 1959, now Patent No. 3,106,499.

Due to the inherent characteristic of aluminum to combine with oxygen, the exposed surfaces of most aluminum alloys rapidly develop an oxide scale, coating or film which manifests a dull and unattractive appearance. The film exhibits a great tenacity for the retention of grease, dirt, and other forms of contamination which reduces still further the reflectance characteristics of the original surface. The exterior skins of aircraft, for example, which are generally formed of aluminum or aluminum alloys, in operation, become quite dull in appearance, and where a number of different alloys are used for the several parts and assemblies, the aircraft, as a whole, may present a varying patch work of dull and unattractive shades of gray. It is customary, however, for aircraft manufacturers to require that the exterior skins present a uniform texture and appearance and heretofore, in practice, this usually has been accomplished by rubbing and buffing the surfaces of the skins by hand.

It is also customary in the manufacture of many aircraft assemblies, to electrically weld two or more sheet metal parts together which are made of alloys composed mainly of aluminum. It has been found that thin films of oil, dirt and/or metallic oxide on the faying surfaces of the parts to be joined obstruct the passage of the electric current and cause poor welds. The electrical surface resistance increases markedly upon the formation of the oxides and contaminants on the faying surfaces of the parts to be welded and may prevent their welding altogether. In order to secure spot welds of high strength, it is usually required that the surface or contact resistance between the parts should not exceed 100 microhms.

It is also customary in the manufacture of many aircraft assemblies to employ fusion welding in the joining together of two or more sheet metal parts which are made of alloys composed mainly of aluminum. As aforementioned, the corrosion resistance of aluminum is derived from an oxide of aluminum which forms rapidly on all exposed surfaces. In order to make a sound strong weld, this oxide must be removed as well as any grease, oil, or dirt which may be on the surfaces to be welded.

in preparing for welding, it usually has been the practice to scrub the parts with a stiff wire brush and hot water, or if any oil or grease is present, with a non-flammable .solvent such as sodium hydroxide or tri-sodium phosphate in hot water. In welding the parts, a specially prepared flux is necessary to remove the aluminum oxide which forms rapidly on the molten weld metal. The flux combines chemically with the aluminum oxide to form a greatly impairs and reduces the strength of the joint.

It is also the practice following working or reworking of aircraft parts and assemblies in the form of aluminum sheet stock, extrusions, forgings, etc., to clean, polish, or otherwise treat the surfaces and thereafter apply chemical conversion coatings, or anodic films, or the like, to

protect the exposed surfaces against corrosion during periods of storage or between work operations on the parts and assemblies. In many cases it may be necessary to apply, remove, and reapply such protective coatings several times during the work process. The presence of surface oxides and contaminants, however, seriously impairs the ability of the protective coatings to adhere to and resist corrosion of the surface metal. Many of the etching solutions employed to remove chemical conversion coatings, or the like, or otherwise prepare the metal surfaces for work operations or application of protective coatings thereon, do not produce a clean bright surface after being applied to the metal, or the etches may fail to thoroughly clean certain surface areas where the parts and assemblies have complex and intricate configurations.

Consequently, the quality of the work operation and of the protective coating applied are both frequently impaired by the etching solutions employed to prepare the metal surfaces. So-called spotweld etches, for example, employed to prepare aluminum surfaces for spot welding, while producing satisfactory electrical resistance for welding, due to too drastic etching action, may leave the surface with a dull appearance or coated with a film of some undesirable chemical product which resists adherence of a protective coating which may subsequently be applied.

Various methods heretofore used or proposed for removing the objectionable oil, dirt and metal oxide films, and the chemical conversion coatings and anodic films, have generally employed mechanical or manual abrasion, electro-chemical means, or chemical solutions. Manual bufiing and polishing is laborious, tedious, and time consuming and, because of the human element involved, the effects of texture, color and gloss are non-uniform from one surface area to another. In addition, the abrasive action removes excessive metal, scratches the surface, and leaves a visible pattern of the abrasive marks and strokes.

Electrolytic burnishing or polishing methods are known to produce surfaces having a high reflective capacity on aluminum and aluminum alloys. Electrolytic methods, .however, have increased initial and maintenance costs imposed .by the expenditure of electrical energy in the process. Chemical solutions including the so-called bright dip solutions have also been tried and used with varying degree of success, but these, in common with the electrolytic solutions, have exhibited marked tendency to attack and etch the metal surface with accompanying loss of metal, to preferentially polish certain alloys while being completely ineffective when used with others, to preferentially attack certain partially disassociated alloy constituents, to generate noxious fumes and odors, and to comprise rare and expensive constituent materials, or substances dangerous to handle. In so-called aqueous solutions, for example, a costly concentrated acid etch may be the predominant ingredient in the solution in which the only water is that which is provided with the acid.

According to the present invention there is provided a process and composition which imparts a handsome luster and mirror-like surface to aluminum and the alloys of aluminum. In its preferred form, the composition consists mainly of water in an aqueous solution formed of ammonium bifluoride, nitric acid and boric acid. The .solution preferably is operated at a temperature of 160- 190 F. with the immersion time being from 5 to minutes. After a period of immersion of 10 minutes at 170 F-., the alloys 'of aluminum undergo negligible loss of weight and dimensional volume and manifest a high metallic luster, uniform texture, and similar col-or. The solution is highly effective in preparing aluminum and its alloys for the reception of chemical conversion coatings or anodic films, or will remove these coatings when this is desired. Thesolution also serves as an excellent pre-spotweld etch for aluminum and its alloys, and when used to prepare parts for fusion welding, provides joints of low porosity without requiring a welding flux.

In practice it has been found that certain trade name brands of materials such as ammonium bifluoride, for example, produce a higher degree of metallic luster than do others, or do the so-called commercial grade materials. It has also been found that the degree of luster obtainable from a nitric-fluoric-boric acid solution formed with the use of commercially pure ammonium bifluoride may be controlled, that is, increased or decreased by the addition to the solution of a particular amino compound taken from the family of water soluble amides and thioamides, or to the liquid concentrate, or dry mix used in forming the solution. Thus, the luster imparted by the bath to the aluminum immersed therein may be increased by the use of an amino compound taken from the family of water soluble amides of which the basic form is carbamide, commonly known as urea. Similarly, the luster may be decreased by the use of an amino compound taken from the group of water soluble thioamides of which the basic form is thiocarbamide, commonly known as thiourea. In some cases itis desirable to produce less luster than would normally be obtainable from the nitric-fluoric-bori-c acid solution without use of the amide additive in order to achieve a generally uniform color effect in fabricating a number of aluminum parts into an assembly where the parts may have been cleaned and polished by dilferent methods and/or may be formed of different alloys which produce different degrees of luster with use of the same polish. I

It is an object of the present invention therefore to provide a new and improved process and composition for cleaning and polishing aluminum and the alloys of aluminum by simpleimmersion in a chemical solution.

Another object is to provide a process and composition for imparting a high gloss on the surface of aluminum and its alloys with negligible loss of metal Weight and dimensional volume.

A further object is to provide a method and composition for treating aloys having a high aluminum content which will remove all surface dirt and metallic oxides and only such minimal amount of surface metal as necessary to enhance the reflectance characteristics of the surface.

Another object is to provide such a treatment which leaves the surface-s of commercial aluminum alloys of many different chemical compositions smooth and glossy with relatively uniform color, texture and reflectance characteristics.

Still another object is to provide a treatment of the type described which may be used elfectively on aluminum. and the alloys of aluminum as apre spotweld etchant.

. Still. another object is to provide a chemical process and solution for preparing aluminum and its .alloys for fusion welding in which 'fluxes are not required during the welding operation and porosity in the welded joint is minimized.

Still another object is to provide a chemical solution which is effective in preparing aluminum and its alloys for the reception of chemical conversion coatings and anodic film-s. Still another object is to provide a solution of this type which is effective in removing chemical conversion coatings and anodic films and, additionally, leaves the surface smooth and glossy and free of all metallic oxides and contaminants.

Another object is to provide an aqueous solution for cleaning and polishing aluminum and its alloys in which inexpensive constituent materials are employed and in which water is used in predominant proportions.

An additional object is to provide a chemical process and composition for cleaning and polishing aluminum and its alloys which uses minor proportions of inexpensive chemicals in an aqueous solution and operates at moderately hot temperatures and relatively short periods of time to produce a high gloss'finish with minimal loss of metal and without the production of obnoxious fumes and odors.

Still another object in a treatment of the type described is to provide compositions in dry mix or liquid concentrate form which may be placed in aqueous sol-ution for the purpose of cleaning and polishing aluminum and its alloys by immersion therein.

Another object resides in the provision of an immersion composition and process for controlling the metallic luster imparted thereby to aluminum and its alloys.

Another object is to provide a nitric-fluoric-boric acid composition and immersion process in which an additive is employed to control, i.e., increase or decrease, the luster obtainable from such composition.

Still another object is to provide an additive to a nitricfluoric-boric acid composition to enable the same, when formed of commercial grade materials, to yield cleaning and polishing effects and results fully equivalent to those obtainable from such composition when formed of preferred brand materials.

Yet another object is to provide a luster controlling additive of this general character which may be used, when desired, to decrease the luster normally obtainable from the aforesaid nitric-fluoric-boric acid composition.

A still further object is to provide the aforesaid luster controlling additive in dry mix, liquid concentrate, or ultimate solution form, together with other composition ingredients making up such form, as used in accordance with the present invention to form the ultimate nitricfluoric-boric acid immersion solution as herein disclosed and claimed.

Still other objects, features and advantages of the present invention will become more clearly apparent as the description proceeds, reference being had, in part, to the accompanying drawings wherein:

FIGS. 1 and 2 are views illustrating two fusion Welded joints as they appear on a photomicrograph and in which the parts comprising the joint depicted in FIG. 1 were cleaned preparatory to weldingin accordance with the process of the present invention and the parts comprising the joint depicted in FIG. 2 were cleaned prior to welding in accordancewith a prior art etching process;

FIGS. 3 and 4 are diagrammatic views representative of cross-sections ofthe welded joints of FIGS. 1 and 2 as they would appear along the lines 3-3 and 44 respectively;

FIGS. 5 and-6 are views illustrating photomicrographs of the welded joints taken at 25 times magnification within the fields of View respectively depicted by the circles of FIGS. 3 and 4;

FIGS. 7 and 8 are end views of halves of an aluminum alloy sheet in which the half depicted in FIG. 7 was treated in accordance with the polishingprocess of the present invention after both halves were treated with an alkaline cleaner;

FIGS. 9 and 10 are views respectively depicting the surface conditions of the plates of FIGS. 7 and 8 as they appear on a pair of photomicrographs taken at 500 times magnification within the fields of View respectively depicted by the circles of FIGS. 7 and 8;

FIG. 11 is a view illustrating in tabular from the differences in sheet thickness resulting from treatment of CONTROL and TREATED specimens of various alumi-' num alloys in which the TREATED and CONTROL specimens have been treated in the same manner as the plates of FIGS. 7 and 8 respectively, and I FIG. 12 is a graph illustrating the effect of immersion time on the relative gloss obtained from representative aluminum alloys when polished in the solution of the present invention.

According to the present invention, a prefered threeconstitutent acid composition composed predominantly of water with minor proportions of nitric, hydrofluoric, and boric acids, and an alternative two-constituent acid composition composed predominantly of Water with minor proportions of nitric and fluoboric acids are prepared which, in practice, produce exceptional cleaning and polishing effects, particularly on aluminum alloys, although other metals such as titanium are effectively cleaned thereby. Although the alternative aqueous solution may be prepared with the two-constituent acid composition, the cleaning and polishing effects obtained therefrom are substantially the equivalent of that obtained from use of the three-constituent acid composition for the reason that the fluoboric acid in solution breaks down and forms hydrofluoric and boric acids. It is preferable, however, that a quantity of boron calculated as boric acid, be in excess of the amount required for reaction with all of the fluorine present for reasons hereinafter more fully to appear. To this end, boric acid or its water soluble salts may be added to the twoconstituent acid solution.

In the preferred three-constituent acid composition, the solution is prepared using ammonium bifluoride (NH F-HF) which, in solution, forms hydrofluoride acid (HF) and ammonia (NI-l In its most desired form, the composition consists of a solution composed of 0.2 gram of ammonium bifluoride, 1.0 gram of boric acid (H BO and 3 milliliters (mls.) of nitric acid (42B.) (specific gravity 1.42) dissolved in 100 mls. of water.

The concentration and constituent formulation of the salts and acids may be varied within certain limits from the optimum as represented in the above formulation, as may be seen in the following examples.

HNO mls 2.0-5 .0

6 Example IV I Per 100 mlst v of aqueous solution Alkali metal fluoride (NaF-KF) (calculated as NH F-HF) grams 0.2-0.5 Alkali metal or ammonium borates (Na B O NH HB O (calculated as H grams 1.010.0

HNO mls 2.05.0

From the foregoing, it will be apparent that a weight to volume ratio of about 10:1 exists between the nitric acid and the ammonium bifiuoride. When this ratio is observed there will be no smut formed on parts treated in the solution. It will also be apparent that the boric acid may vary from 2 to 50 parts by weight to one part by weight of the ammonium bifiuoride. The boric acid and alkali metal fluorides calculated as ammonium bifluoride in these proportions may be prepared as a dry powdered mixture from commercial grade materials and marketed as a dry mix or as a solid in order to promote economies in packaging and shipping.

In preparing the solution, the dry mix is dissolved in water and the nitric acid is preferably added last. The nitric acid in the solution composition should have a specific gravity of 1.42 and any commercial grade of dilution may be employed provided the required weight of acid is obtained in the resulting solution and due compensation is made for any difference in water volume. The approximate range of materials in solution may thus be:

Percent by Weight of aqueous solution Alkali Metal Fluoride (calculated as NH F-HF) 0.2 to 0.5 H 80 1.0 to 10.0 HNO 2.0 to 5.0

A preferred dry mix comprises 1 part by weight of alkali metal fluoride (calculated as ammonium bifluoride) to 5 parts by weight of boric acid. Thus, a preferred solution will be formed when the equivalent of the preferred ultimate 2% volume of nitric acid, is added to form about 96 mls. of water having alkali metal fluoride calculated as 0.2 gram NH F-HF and 1.0 gram H BO dissolved therein. Ammonium bifluoride and boric acid solids having these weights will not dissolve in 2 mls. of nitric acid without the addition of approximately 10 mls. of water. Accordingly, the preferred solution may be concentrated only to the extent that at least 10% of the 96% water that goes into final solution is retained in the concentrated solution in order to get the solids (Nl-L F-HF and H BO to dissolve with the nitric acid. This concentrated solution will then comprise:

NH EHF grams 0.2 H BO do 1.0 HNO rnls 2 H O mls 10 and may be used for each 100 mls. of ultimate solution. It therefore only becomes necessary to add 86 mls. of water to the liquid concentrate to form each 100 rnls. of solution and bring the same to the desired formulation. The concentrated solution conveniently may be marketed in polyethylene bottles, or the like, to prevent deterioration from the hydrofluoric acid.

The materials of the concentrated solution may fall within the following ranges:

Percent of ultimate solution by weight NH4FHF .2 to .5. 11,30 1.0 to 10.0. HNo 12.0 to 25.0(16

to 20% acid).

The solution, the liquid concentrate, or the dry mix, as the case may be, may be prepared from hydrofluoric (HF), fluoboric (HBF and hydrofiuosilicic acids (H SiF and the water soluble salts thereof, provided that the total weight of fluoride ion is comparable to that hereinbefore specified for the optimum formulation. Thus, in place of the foregoing fluo acid-s, water soluble fluo compounds such as alkali metal or ammonium fluorides, fluosilicates, and fluoborates may be used in proper amounts to liberate an equivalent concentration of hydrofluoric acid. These may include among others sodium and potassium fluoride (NaF, KF), sodium fluosilicate (Na SiF sodium fluoborate (NaBF ammonium bifluoride (NH F.HF), ammonium fluoborate (NH BF and ammonium flue-silicate (NH SiF Similarly, the boric acid may be replaced by its water soluble salts, such as the alkali metal and ammonium borates including sodium borate (Na B O (Borax) and ammonium borate (NH H.B O in amounts capable of supplying boric acid of the required concentration in water.

The two-constituent acid composition aforedescribed may simply be prepared by adding nitric acid and fluoboric acid, or the water soluble salts thereof, to the proper amount of water by observing the fluoride and borate ion concentrations required in the three-constituent acid composition, the nitric acid being in the same proportions and the fluoboric acidor its salts, as the case may be, supplying both fluorine and boron required in the solution. It may be preferable to calculate the quantity of fluoboric acid, or one of its water soluble salts, as the case may be, in terms of the fluoride ion required, and making up any deficiency, or augmenting the supply, of boron in the solution by the addition of boric acid in order preferably to keep the borate ion concentration in excess of that required for buffering the fluoride ion, since the boric acid may be lost, in part, through volatilization and this, in turn, may result in too severe activity of the hydrofluoric acid on the metals under treatment in the solution.

Nitric and fluoboric acids or nitric acid and one or more water soluble fluoboric salts may separately be added to the proper amount of water or the same added in the form of a concentrated solution of these compounds. For example, a 100 ml. aqueous solution of nitric and fluoboric acids may consist of:

Percent volume of 100 ml. HBF 1.3 to 3 HNO It will be apparent that concentrated solutions of these acids in these proportions, or equivalent concentrated solutions of the nitric acid with suitable fluoborates may be prepared in form suitable for marketing for the purposes of this invention, as 'hereinbefore described.

In the case of each of the foreging solutions, which are operated as so called hot solutions, the-acid composition is critical both as to the components and their proportions, particularly with respect to the relative concentrations of the fluoride and borate ions, and is also critical as to the range of operating temperature.

In these solution, the hydrofluoric acid is considered to be the sole etchant and, by itself, will vigorously attack aluminum and its alloys. A passivating agent or buffer is required to retard and, inhibit this etching action in order to minimize the attack on the surface metal and thus remove only that amount of metal essential for cleaning and/or polishing, as the case may be. The borate ion in the solution provides this retarding and inhibiting effect such thatlittle or no visible action of the acid on the metal may be observed when the solution is at ambient temperature (70 F.) and contains the proper concentrations of the fluoride and borate ions.

As ionization of the hydrofluoric and boric acid solution (nitric acid being absent) is increased by raising the temperature of the solution up to the order of 200 F., the action of the acid on the metal increases but is insuflicient to adequately polish the metal or otherwise serve the purposes of the invention. An oxidizing agent is required to oxidize the surface contaminants and make them soluble in the solution. This function is supplied by the nitric acid which apparently attacks the surface contaminants, releasing nitric oxide in the process. This action of the nitric acid is inappreciable below F. As the temperature is increased above 160 F., the nitric acid apparently breaks down and an evolution of hydrogen is visi'ble. Nitric oxide also is released from the solution and, although this is in such small volume as to be hardly detectable and therefore unobjectionable, the fact that the nitric acid is functioning may be detected near the surface by the slight odor of the nitric oxide gas which is escaping from the solution.

As aforementioned, the hydrofluoric, boric, nitric acid solution may be operated at temperatures of the order of 200 F. and upwards. The loss of the boric acid through volatilization at the elevated temperatures, however, makes an operating range of the order of 160 to 190 F. preferable, a specific preferred operating temperature in this range being F. It will also be understood that in operation, the solution is maintained at this temperature and such additional amounts of nitric acid and fluoride and/ or borate ion must be added from time to time as necessary to maintain the desired concentration of the acid constituents.

The aforedescribed hot solutions when employed to clean and polish aluminum and its alloys by simple immersion therein produce superior results and are effective on all aluminum alloys provided the alloying constituents at the surface of the metal do not exceed the order of 5 to 6% of the alloy. The process and composition has been found to be effective with the following widely used aluminum alloys in all physical conditions and having the approximate compositions noted in percent by weight:

Type 2024 Type 5052 Type 6061 Type 7075 0.1 .15 to .40-"- 1.2 to 2.0.

Ti 0.2, A1

balance.

Referring now to the drawings, and first more particularly to FIG. 11, there is shown a tabulation of aluminum alloys in which a test sample (approximately 3 x 4 inches) for each alloy depicted was divided along the 4 inch axis to provide the designated CONTROL and TREATED specimens. By reason of this arrangement, the initial gauge thickness of both specimens was substantially the same. The control and treated specimens were both subjected to a cold alkaline cleaner to remove foreign matter such as would affect the optical micrometer thickness measurements, and the treated specimens thereafter were subjected to the immersion polish and cleaner of the present invention for 10 minutes at 170 F. The reduction in gauge measurements providing the minimum, maximum, and average gauge readings depicted were then made and the difference noted. It will be appreciated that in each case there is either no reduction in gauge, or such reduction as occurs is negligible or at the most, as in the case of 2024T81, is of the order of less than 1.5% of the average gauge of the untreated specimen. The increase in gauge thickness has no known significance and may be attributable to errors in the use of the optical micrometer.

End views of the TREATED and CONTROL specimens for the 6061-T6 alloy are illustrated in FIGS. 7 and 8 respectively. Photomicrographs taken at 500 times magnification within the fields of view indicated by the circles 20 and 21 in FIGS. 7 and 8 are depicted in FIGS. 9 and 10 respectively. The photomicrographs comparatively reveal that the surface of the treated specimen is nearly fiat whereas that of the control specimen is relatively rough having appreciable crests 22 and valleys 23. Accordingly, assuming the light to be directed generally normal to the surfaces of the specimens, there is no breaking up of the incident rays of light as indicated by the generally parallel lines 24 depicted in FIG. 9 and, hence, a. high degree of reflectance is obtained from the polished surface of the treated specimen. By contrast, many of the incident rays of light striking the surface of the control specimen are angularly displaced as depicted by the angular lines 25 in FIG. 10, this being due to the relatively rougher surface of the control specimen. Hence, the reflectance characteristics of the control specimen surface are substantially impaired.

The relative reflectance or gloss of aluminum alloys treated with the cleaning and polishing solution of the present invention in relation to the immersion time in minutes is disclosed in FIG. 12 wherein it will be seen that relatively higher gloss is obtained from the aluminum clad alloys 2024 and 7075 than from the unclad alloy 6061. It may also be noted that the greatest increase in gloss apparently takes place in the first minutes and that little or no increase occurs after minutes, this being generally the case for normal operating conditions and specifically for the conditions under which the alloys disclosed in FIG. 12 Were treated, the temperature of the three-constituent acid solution employed for this purpose having been maintained at 170 F. and optimum proportions of the solution constituents employed. Hence, under optimum operating conditions, the optimum immersion time may be taken to be from 5 to 10 minutes.

By way of contrast between the two and three-constituent acid compositions of the present invention, a solution of about 2% by volume of nitric acid and about 1.3% by volume of fiuoboric acid, the balance being water, was heated to 170 F. and a 2024 specimen immersed in the solution for seven minutes produced a maximum gloss reading of 58 on the scale depicted in FIG. 12.

It will be understood that the gloss values depicted in FIG. 12 are relative only. These measurements were made using a Gardner 60 Glossmeter equipped with a No. 0.9 B & L Filter, the Glossmeter first being calibrated through use of a polished black Carrara glass providing a standard reading of 94 on the meter. The interposition of the filter cut the reading approximately 87 gloss units to a reading of about 7 on the meter and, hence, the reading of 40 for 606l-T6 at about 4 /2 minutes would correspond to a true gloss reading of approximately 87 additional gloss units or 127.

The solutions of the present invention are also effective in preparing aluminum and its alloys for fusion welding such as by inert gas shielded arc welding, this being illustrated pictorially in FIGS. 1 to 6 wherein FIGS. 1 and 2 depict a pair of fusion welded joints 26 and 27 as seen on a single photomicrograph. The parts forming the welded joint in each case were 6061 alloy, the parts 28 and 29 of FIG. 1 having been cleaned by immersion in the optimum solution of the present invention and under optimum conditions as hereinbefore specified. The parts 3% and 31 comprising the welded joint of FIG. 2 were cleaned prior to welding with a prior art cleaner and etch such as that disclosed in my copending application for Process and Composition for Treating Aluminum Alloys, Serial No. 709,442 filed January 17, 1958, now Patent No. 3,003,896.

The welded joints 26 and 27 were both formed by the Heliarc process and aluminum alloy welding rod 438 was used in each case, the usual puddling action being employed. The 438 alloy has the following composition in percent by weight:

438 welding rod:

Copper .3.

Iron .8.

Silicon .45-6.

Manganese .05. Zinc .1.

Titanium .2

Aluminum Balance.

The welded joint 26 depicted in FIG. 1 was made without the use of flux whereas flux was employed in forming the joint of FIG. 2 as evidenced by the darkened areas 32 due apparently to the capillary action or flow induced by the flux. Also in evidence in FIG. 2 are the relatively light areas 33 adjacent the darkened areas 32, this being due to the wire brushing of the welded joint 27 which is essential for removal of the flux following completion of the Welding operation.

FIGS. 3 and 4 depict cross-sections of the welded joints of FIGS. 1 and 2 and indicate by the circles 34 and 35 the fields of view within which the photomicrographs 36 and 37 of FIGS. 5 and 6 were taken at 25 times magnification.

By comparing the two photomicrographs 36 and 37 it may be observed that the region 38 of the welded joint as depicted in FIG. 5 is free of any evidence of porosity whereas the comparable region 39 of FIG. 6 is replete with large holes 40 which evidence a high degree of porosity and consequently a welded joint of greatly reduced strength.

The cleaning and polishing solutions of the present invention are also effective as a pre-spotweld etch and are generally able to accomplish the same order of reduction of electrical surface resistance as obtained in the use of my process and composition disclosed and claimed in the aforesaid application, Serial No. 709,442 wherein the parts to be prepared for spot welding are first immersed in a nitric-chromic acid solution for three minutes, then after rinsing, immersed in an ammonium bifiuoride-citric acid solution for two minutes, and again after rinsing, immersed in the nitric-chromic acid solution for two minutes. This prior art process has the advantage of being operable at ambient temperature. However, the hot solutions of the present invention have the advantage that the same order of resistance reduction may be obtained while requiring only a single rinse following the single immersion treatment. In addition, a highly polished surface is obtained which is receptive to the application of conversion coatings and anodic films. The hot solutions are also effective in removing these coatings and films preparatory to re-application thereof or in preparing parts for further work operations thereon. For example, a certain part may undergo the following operations:

(1) Vapor degrease involving immersion in or treatment with the vapor of trichloroethylene, ethylene dichloride or other known solvent to remove any surface oil or grease film.

(2) Clean and polish in the heated nitric-fluoric-boric immersion bath of the present invention.

(3) Alodize by application of a chemical conversion coating such as Alodine 1000 which is a commercially available transparent film.

(4) Store for indefinite periods. In the case of parts scheduled for later spotwelding, for example, the parts may be stored for upwards of 30 days without increasing the electrical surface resistance above 25 microhms.

(5) Re-work or finish. In the presumed spot welding example, this operation may be performed without removal of the Alodine 1000 and, following spot-welding,

the processing of the parts may be considered to be comthis being done, for example, in the case of detail parts which require a final protective coating such as that afforded by commercially available Alodine 1200.

(7) Alodize by application of Alodine 1200.

When desired, parts formed of aluminum and its alloys such as sheet stock, extrusions, etc., which have been cleaned and polished in the heated nitric-fluoric-boric acid immersion bath of the present invention, may be further treated for from 3 to 5 minutes in a nitric-chromic bath as aforementioned, such a bath preferably comprising an aqueous solution of the composition:

Nitric acid (HNO 15 to 25% by weight. Chromic acid (CrO 0.5 to 1.5% by Weight. Water (H O) Balance.

wherein the optimum content of nitric acid is approximately 22 percent. It is not necessary to heat this solution since it is effective within the temperature range of 70 to 90 F. This solution dissolves any small particles of smut which may have formed on the parts while in the nitric-fluoric-boric bath and brightens the metallic surfaces without impairing the specular gloss imparted thereto in the previous cleaning and polishing treatment.

It is desirable that the aforedescribed cleaning and polishing results be obtainable in the use of commercial grade materials in forming the immersion solution. It has been noted in actual practice, however, in the use of materials for formulating the solution that superior polishing results may be achieved in the use of certain brand name types of the materials available on the market. Certain brand name types of ammonium bifluoride, for example, produce a higher degree of metallic luster than is obtainable in the use of commercial grades of this material in the same formulation.

It has been theorized that nitrites which form in the ultimate solution in its use, in turn, yield nitrous acid which tends to slow up the polishing action. It is further theorized that the brand name ammonum bifiuoride materials have non-identifiable additives or trace materials which tend :to disperse the nitrites and thus produce superior polishing results.

In accordance with this theory, an additive, in the optimum form of urea, otherwise known as carbamide CO (NH is added to the aforedescribed formulations, whether dry mix, liquid concentrate, or ultimate solution, this being done in view of the known ability of urea in dispersing nitrites. This is evidenced by the following equation:

from which it may be noted that the nitrous acid is broken down to form nitrogen and carbon dioxide gas which leaves the solution. Although it is not actually known to be the case, the evolution of the carbon dioxide gas is believed to produce a beneficial mechanical polishing eifect on the materials being treated in the immersion solution.

In accordance with the foregoing, various examples of ultimate solutions, prepared from commercial grade materials, will have the following formulations:

Example I Per 100 mls. of aqueous solution HF grams 0.15-0.35 H BO do 1.0-10.0 H'NO mls 2.0-5.0 CO(NH grams 0.1-1.0

' Example 11 Per 100 mls. of aqueous solution H SiF grams 0.55-1.25 H.130, do LOO-10.00- HNO mls 2.0-5.0 CO(NH "grams... 0.1-1.0

12 Example Ill Per 100 mls. of aqueous solution l-IBF and H 30 Equivalent to 0.15-0.35 gm. HF

and 1.0-10.0 gms. H COQNHQ Oil-1.0 gm.

In lieu of urea CO(NH any amino compound se lected from the family of water soluble amides may be used and in the same amount. Thus, the brightening additive may be selected from the following family of water soluble amides of which the basic form is carbamide (urea):

Carbamide (urea), NH CONH Acetamide, CH CONH Propionamide, C H CONH Butyramide, C H CONH Valerie amide, C H CONH Caproamide, C H CONH Heptarnide, CsHmCONHz. Caprylamide, C H CONH Nonanamide, C H CONI-I An amino compound, as referred to hereinabove, is defined as one pertaining to or containing the NH radical, otherwise known as the amino group. Thus, the thioamides of which the basic form is thiocarbamide (thiourea) are amino compounds.

The water soluble thioamides may be used in accordance with the additive formulation of the present invention to produce decreases in metallic luster from that ob tained in the use of commercial grades of materials to form the ultimate nitric-fluoric-boric acid solution. This follows from the dulling effect of' the hydrogen sulfide H 8 which forms in the solution and, in turn, reacts with the metals therein. Copper sulfide Cu S, for example, is black and will tend to form a smut on the surfaces being treated in the solution. The development of the hydrogen sulfide in the solution follows from the fact that reactions of thiourea are analogous to those of urea as is evidenced by the following equation:

An optimum ultimate solution formed of commercial grade materials for producing decreases in metallic luster will have the following formulation:

Example I Per mls. of aqueous solution HF "grams" 0.15-0.35 H BO do 1.0-100 HNO rnls 2.0-5.0 CS(NH grams 0.1-1.0

Example II Per 100 mls. of aqueous solution H SiF grams 0.55-1.25 H BO do LOO-10.00 HN0 mls 2.0-5.0 CS(NH grams (ll-1.0

Example III Per 100 mls.

of aqueous solution HBF and H BO Equivalent to 0.15-0.35 gm. HF

and 1.0-10.0 grams H 30 CS(NH 0.1-1.0 gram.

Any of the water soluble thioamides may be used in lieu of thiourea CS(NH thus, the dulling additive may be selected from the following family of water soluble thioamides:

Thiocarbamide (thiourea), NH CSNH Thioacetamide, CH CSNH Thiopropionamide, C H CSNH Thiobutyrarnide, C H CSNH 13 Thiovaleric amide, C H CSNH Thiocaproamide, C H CSNH Thioheptamide, C H CSNH Thiocaprylamide, C H CSNH Thiononamide, C H -,CSNH

The additive as aforedescribed, thus broadly, may be used together with the commercial grade materials used in formulating the ultimate nitric-fluoric-boric acid solution to control the degree of metallic luster which may be obtained from use of the ultimate solution, that is, the luster may be increased (brightening effect) or selectively, the luster may be decreased (dulling elfect). As noted above, this is accomplished in either case by selecting for use as the additive, an amino compound from the family of water soluble amides and thioamides. When an increase in luster is desired, an amide is selected. Conversely, when a decrease in luster is desired, a thioamide is selected.

The additive formulation as a liquid concentrate may take the following forms:

BRIGHTENING Example 1 Per 100 mls. of aqueous solution NH F.HF grams 0.2-0.5

H BO do 1.0-10.0

HNO mls 2.0-5 .0

CO(NH grams-.. 0.1-1.0

Example 11 Per 100 mls. of aqueous solution NH F.HF grams .2 H BO do 1.0 HNO mls 2 CO (NI-1 grams 0.1 H 0 mls 10 Example III Percent of ultimate Per 100 mls.

of aqueous solution NH F.HF gram .2 H3BO3 'dO HNO mls 2.0 CS(NH gram 0.1 H 0 rnls 10 Example 111 Percent of ultimate solution by Weight NmRHF 0.2 to 0.5. H3BO3 1.0 to 10.0. HNO 12.0 to 25.0 (16to 20% acid). CS(NH2)2 0.1 to 1.0.

The additive formulation as a dry mix (the amides and thioamides being available in solid form) may take the following forms:

BRIGHTENING Percent by weight of ultimate aqueous solution including 2.0-5.0% HNO Alkali metal fluoride (calculated as NH F.HF) 0.2 to 0.5 Alkali metal or ammonium borates (calculated as H 30 1.0 to 10.0 CO(NH 0.1 to 1.0

DULLING Percent by weight of ultimate aqueous solution including 2.0-5.0% HNO Alkali metal fluoride (calculated as NH RHF) 0.2 to 0.5 Alkali metal or ammonium borates calculated as H BO 1.0 to 10.0 CS(NH 0.1 to 1.0

Although the present invention is directed primarily to the treatment of articles formed of aluminum and its alloys by immersion in a heated bath solution, it will be understood that the articles may effectively be cleaned and polished by brushing, spraying, or swabbing the same with the solution for the required period of time to obtain the desired gloss provided, of course, that the required temperature and proportions of the solution are maintained.

The novel principles of this invention transcend the scope of the invention as suggested or implied by the several embodiments hereinbefore described, and the invention may be embodied in other forms or carried out in other ways which have been conceived and reduced to practice during the course of this development, without departing from the spirit or essential characteristics of the invention. The embodiments disclosed herein therefore are to be considered as in all respects illustrative and not restrictive, the scope of the invention being indicated by the appended claims, and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein.

Having thus described my invention, what I claim as new and useful and desire to secure by Letters Patent is:

1. A dry mix additive to form a hot aqueous solution containing 2.0-5.0% nitric acid for cleaning and imparting high gloss to the surfaces of aluminum and its alloys consisting of urea, a fiuo compound selected from the group consisting of water soluble alkali metal fluorides, fiuosilicates and fiuoborates and a boric compound selected from the group consisting of boric acid and water soluble salts thereof, said urea and boric and fiuo compounds being in amount equivalent to form in the solution about 0.1 to 1.0% by weight of urea, 2 to 50 parts by Weight of boric acid to 1 part by weight of hydrofluoric acid calculated as ammonium bifiuoride, said fiuo compound being .in amount such that the hydrofluoric acid calculated as ammonium bifluoride has a volume to weight ratio of about 10 to 1 between the nitric acid and ammonium bifiuoride.

2. An addition agent to be added to water to form a hot aqueous acid bath for cleaning and polishing aluminum and its alloys consisting of a concentrated nitricfiuoric-boric acid composition providing the equivalent of 2.0-5.0 mls. of nitric acid, 0.15-0.35 gram of hydrofluoric acid, 1.0 to 10.0 grams of boric acid, and 0.1 to 1.0 gram of urea in mls. of aqueous solution.

3. A liquid concentrate to be added to water in the preparation of a hot aqueous solution for imparting a high gloss to articles of aluminum and its alloys immersed in said solution and consisting of water in amounts providing upwards of 10.0% of the ultimate solution, urea in amount to form 0.1 to 1.0% by weight of the aqueous solution, nitric acid in amount to form about 2.0-5.0% of the aqueous solution, a fluo compound selected from the group consisting of hydrofluoric, fluorboric, and fluosilicic acids and water soluble fluorides, fluosilicates, and fluoborates, said fluo compound being in amount equivalent to form about 0.15-0.35 gram hydrofluoric acid per 100 mls. of aqueous solution, and a boric compound selected from the group consisting of boric acid and water soluble salts thereof, said boric compound being in amount equivalent to form about 1.0-10.0 grams of boric acid per 100 mls. of aqueous solution. I

4. A hot aqueous solution for chemically cleaning and polishing articles formed of a metal consisting principally of aluminum and formed by the addition to water of a composition consisting of 2.0-5.0 mls. of nitric acid per 100 mls. of aqueous solution, 1.0-10.0 grams of boric acid per 100 mls. of aqueous solution, 0.2-0.5 gram of ammonium bifluoride of commercial grade per 100 mls. of aqueous solution, and 0.1 to 1.0 gram of urea per 100 mls. of aqueous solution.

'5. A hot aqueous solution for chemically polishing articles formed of aluminum and its alloys consisting of 2.0-5.0 mls. of nitric acid per 100 mls. of aqueous solution, 1.0-10.0 grams of boric acid per 100 mls. of aqueous solution, 0.15-0.35 gram of hydrofluoric acid per 100 mls. of aqueous solution, and 0.1 to 1.0 gram of urea .per 100 mls. of aqueous solution.

6. A hot aqueous solution for cleaning and polishing articles formed of aluminum and its alloys consisting of 2.0-5.0 mls. of nitric acid per 100 mls. of aqueous solution, 1.0-10.0 grams of boric acid per 100 mls. of aqueous solution, 0.55l.25 grams of fluosilicic acid per 100 mls. of aqueous solution, and 0.1 to 1.0 gram of urea per 100 mls. of aqueous solution.

7. A hot aqueous solution for chemically cleaning and polishing articles formed of aluminum and its alloys and consisting of about 0.1-1.0% by weight of urea, 25% of nitric acid and fluoboric and boric acids in amounts equivalent to .15 to .35 gram hydrofluoric and 1.0 to 10.0 grams boric acid in 100 mls. of aqueous solution.

8. The process of chemically polishing the surfaces of articles for-med from a metal consisting principally of aluminum which comprises the steps of forming an aqueous solution consisting of from 0.1 to 1.0 gram of urea per 100'mls. of aqueous solution, 2.0 to 5.0 mls. of nitric acid per 100 mls. of aqueous solution, 1.0-10.0 grams of boric acid per 100 mls. of aqueous solution, and 0.20.5 gram of ammonium bifluoride of commercial grade per 100 mls. of aqueous solution, maintainingthe temperature of said aqueous solution between l70190 F. and immersing the articles to be polished in said solution for .a period of from 5 to 20 minutes.

9. The process of treating aluminum and its alloys to clean and impart high gloss to the surfaces of articles formed therefrom which comprises the steps of immersing said articles in an aqueous acidic solution formed essentially of water, and consisting of about 0.1 to 1.0 gram of urea per 100 mls. of aqueous solution, 2.0-5.0 mls. of nitric acid per 100 mls. of aqueous solution, a boric compound selected from the group consisting of boric acid and water soluble salts thereof and in an amount equivalent to form about 1.0-10.0 grams of boric acid per 100 mls. of aqueous solution, and a fluo compound selected from the group consisting of hydrofluoric acid, fluoboric acid, fluosilicic acid, and water soluble fluorides, fluosilicates and fluoborates, said fluo compound being in amount equivalent to form about 0.15- 0.35 gram of hydrofluoric acid per 100 mls. of said aqueous solution, and wherein the treatment is conducted with said aqueous solution at a temperature of about 160-200 F. for from about 5 to 20 minutes.

10. A process for cleaning an article composed of an aluminum-rich alloy which comprises the steps of subi t g he ar icle to theaction of a heated a ueoussolw tion consisting of from 0.1 to 1.0 gram of urea per 100 mls. of said aqueous solution, 2.0-5.0 mls. of nitric acid per 100 mls. of said aqueous solution, 1.0 to 10.0 grams of boric acid per 100 mls. of said aqueous solution, and 0.15-0.35 gram of hydrofluoric acid per 100 mls. of said aqueous solution, rinsing the article with water, and thereafter subjecting the article to the action of a solution consisting essentially of about 22% nitric acid, 0.5% chromic acid, 77.5% water by weight followed by rinsing in water and drying.

11. A process for cleaning an article composed of an aluminum-rich alloy which comprises the steps of subjecting the article to the action of a heated solution consisting essentially of water, urea in about 0.1 to 1.0% by weight of said solution, nitric acid in about 2.0-5.0% by volume of said solution, and fluoboric and boric acids equivalent to about .15 to .35 gram hydrofluoric acid and about 1.0 to 10.0 grams of boric acid per 100 mls. of said solution, rinsing with water, and thereafter treating said article with a solution consisting essentially of about 22% nitric acid, 0.5% chromic acid, 77.5% water by weight.

12. The process for cleaning and preparing articles composed of an aluminum-rich alloy for electrical resistance or fusion welding which comprises the steps of forming a solution consisting of 0.1 to 1.0 gram of urea per mls. of said solution, 2.0-5.0 mls. of nitric acid .per 100 mls. of solution, 1.0-10.0 grams of boric acid per 100 mls. of solution, and 0.2-0.5 gram of ammonium bifluoride of commercial. grade per 100 mls. of solution, maintaining the temperature of the solution between -190 F., immersing said articles in the solution for a period of from 5 to 20 minutes, rinsing the articles in water, and thereafter immersing the articles in a nonetching nitric-chromic acid bath consisting essentially 'of about 22% nitric acid, 0.5% chromic acid, 77.5% water by weight, followed by rinsing in water and drying.

13. The process of removing chemical conversion and anodic films from the surfaces of articles formed principally of aluminum comprising the steps of forming an aqueous solution consisting of about 0.1-1.0 gram of urea, 2.0-5.0 mls. of nitric acid, 1.0-10.0 grams of boric acid, and 0.20.5 gram of ammonium bifluoride of commercial grade dissolved in 100 mls. of said aqueous solution, maintaining the temperature. of the solution between 190 F., immersing said articles in said aqueous solution for a period of from 5 to 20 minutes, rinsing said articles with water following removal from said solution, and drying said articles following rinsing.

14. A liquid concentrate to be added to water in the preparation of a hot aqueous solution for imparting a high gloss to articles of aluminum and its alloys immersed in said solution and consisting of the following percentages of ultimate solution by weight:

Percent NH CONH 0.1 to 1.0 HF (calculated as NILRHF) .2 to .5 H3BO3 to HNO 12.0 to 25.0

(16 to 20% acid) 15. A dry mix additive to form a hotaqueous solution including 2.05 .0 percent nitric acid for cleaning and imlent to form in the solution about 2 to SOparts by weight alloys and. consisting of urea in amountto form 0.1 to 1.0%' by Weight of said aqueous solution, a fluoboric compound selected from a group consisting of water soluble fluoborates in amount equivalent to 0.15-0.35 gram of hydrofluoric acid per 100 mls. of aqueous solution, and a'boric compound selected from the group consisting of boric acid and Water soluble salts thereof, said fluoboric and boric compounds being in amounts equivalent to form in the solution about 2 to 50 parts by weight of boric acid to 1 part byweight of hydrofluoric acid ca culated as ammon um bifiuoride.

16. A dry mix additive to form a hot aqueous solution including 2.0-5.0 percent nitric acid for cleaning and imparting high gloss to the surfaces of aluminum and its alloys and consisting of urea in amount to form 0.1 to 1.0% by weight of said aqueous solution, an alkali metal fluoride calculated as ammonium bifluon'de and a boric compound from the group including boric acid and its salts and calculated as boric acid, said fluoride and boric compounds being in the ratio of 1 part by weight ammonium bifluoride to from 2. to 50 parts by Weight of boric acid, said fluoride being in amount suflicient to provide from 0.15-0.35 gram of hydrofluoric acid in the ultimate solution.

References Cited by the Examiner UNITED STATES PATENTS Howard et a1 156-22 X Murphy 15621 Bellinger 156-21 Greisl et a1. 204140.5

Kendall 15621 EARL M. BERGERT, Primary Examiner.

Examiners. 

1. A DRY MIX ADDITIVE TO FORM A HOT AQUEOUS SOLUTION CONTAINING 2.0-5.0% NITRIC ACID FOR CLEANING AND IMPARTING HIGH GLOSS TO THE SURFACES OF ALUMINUM AND ITS ALLOYS CONSISTING OF UREA, A FLUO COMPOUND SELECTED FROM THE GROUP CONSISTING OF WATER SOLUBLE ALKALI METAL FLUORIDES, FLUOSILICATES AND FLUOBORATES AND A BORIC COMPOUND SELECTED FROM THE GROUP CONSISTING OF BORIC ACID AND WATER SOLUBLE SALTS THEREOF, SAID UREA AND BORIC AND FLUO COMPOUNDS BEING IN AMOUNT EQUIVALENT TO FORM IN THE SOLUTION ABOUT 0.1 TO 1.0% BY WEIGHT OF UREA, 2 TO 50 PARTS BY WEIGHT OF BORIC ACID TO 1 PART BY WEIGHT OF HYDROFLUORIC ACID CALCULATED AS AMMONIUM BIFLUORIDE, SAID FLUO COMPOUND BEING IN AMOUNT SUCH THAT THE HYDROFLUORIC ACID CALCULATED AS AMMONIUM BIFLUORIDE HAS A VOLUME TO WEIGHT RATIO OF ABOUT 10 TO 1 BETWEEN THE NITRIC ACID AND AMMONIUM BIFLUORIDE.
 14. A LIQUID CONCENTRATE TO BE ADDED TO WATER IN THE PREPARATION OF A HOT AQUEOUS SOLUTION FOR IMPARTING A HIGH GLOSS TO ARTICLES OF ALUMINUM AND ITS ALLOYS IMMERSED IN SAID SOLUTION AND CONSISTING OF THE FOLLOWING PERCENTAGES OF ULTIMATE SOLUTION BY WEIGHT: PERCENT NH2CONH2 0.1 TO 1.0 HF (CALCULATED AS NH4F.HF) .2 TO .5 H3BO3 1.0 TO 10.0 HNO3 12.0 TO 25.0 (16 TO 20% ACID) 